Photoimageable compositions are used for various purposes in the manufacture of printed circuit boards or the like. Photoimageable compositions in the form of a primary imaging photoresist are used in the formation of the printed circuitry itself. In a typical procedure, a panel for forming a printed circuit board comprises a thin layer of metal, e.g., copper, covering a non-conductive substrate. A layer of photoresist is applied over the metal layer. The photoresist is exposed to patterned actinic radiation, e.g., by passing illumination through artwork having an opaque/transparent pattern. The exposed photoresist is then developed by exposure to a solution or solvent which, depending upon the type of photoresist, washes away either the exposed or unexposed portion of the photoresist, leaving a patterned layer of either unexposed or exposed photoresist. The panel can then be etched with a solution which removes the metal layer from those portions of the plate from which photoresist has been removed. Subsequent to etching, the remaining photoresist layer is generally stripped away. Alternatively the panel can be pattern plated, whereby the areas devoid of photoresist are electrolessly or electro-chemically plated with copper or the like, the resist is stripped, as above-mentioned, and the newly exposed areas are then removed by etching, leaving behind the pattern plated areas.
Other photoimageable compositions are used for forming solder masks in a similar manner. Such a composition is used to form a hard, permanent layer which overlies printed circuitry except in those portions of the printed circuitry to which solder is to be applied.
Photoimageable compositions may be applied to a panel in liquid form and then either allowed to dry or to partially cure so as to form a semi-stable layer.
Alternatively, the photoimageable composition may be used to form a dry film comprising a layer of photoimageable composition on a cover sheet of support material. The layer of photoimageable material is subsequently transferred from the support sheet to a surface of the panel board.
Dry films comprise the layer of photoimageable material and the cover sheet of support material. The support material is somewhat flexible but has sufficient rigidity to provide structure to the layer of photoimageable material. Typically the cover sheet is formed of a polyester, such as polyethylene terephthalate (PET), such as that sold as MELINEX.sup.R. As the photoimageable layer is to be transferred from the cover sheet, a release agent may be applied to the surface of the cover sheet which carries the photoresist layer. It is typical to also provide a protective sheet, e.g., a sheet of polyethylene, on the surface of the photoimageable layer opposite the cover sheet The protective sheet, in addition to protecting the photoimageable layer, better enables the dry film to be rolled into a reel. The protective sheet is removed from the photoresist layer prior to application of the dry film to the circuit board. U.S. Pat. No. 4,530,896 and U.S. patent application Ser. No. 07/210,194 filed Jun. 20, 1988 the teachings of each being incorporated herein by reference, describe dry films in which a top coat is interposed between the cover sheet and photoimageable layer, which top coat is selectively adherent to the photoimageable layer (relative to its adherence to the cover sheet). The top coat serves to protect the photoimageable layer from mechanical damage and from oxygen after removal of the cover sheet prior to exposure to patterned actinic radiation. Other examples of dry films can be found in U.S. Pat. Nos. 3,887,450, 4,539,286 and 4,610,951, the teachings of each being incorporated herein by reference.
The present invention is generally directed to photoimageable compositions and dry films formed therefrom which are used in the manufacture of printed circuit boards, including those in which the photoimageable composition is a primary imaging photoresist and those in which the photoimageable composition is a solder mask-forming composition. The invention is applicable to dry films in which the photoimageable layer is in direct contact with the cover sheet (in which case the photoimageable layer is generally exposed through the cover sheet) and those in which a top coat is interposed between the photoimageable layer and the cover sheet (in which case the cover sheet is generally removed prior to exposure of the photoimageable layer).
A most important property of any photoimageable composition to be applied to a metallic surface (generally copper of a board is the degree of adhesion that the photoimageable composition has to the metal. Many defects in printed circuit board manufacture can be attributed to inadequate adhesion of the photoimageable composition to the metal. It is known to add agents to photoimageable compositions for the specific purpose of enhancing adhesion of the photoimageable composition to metal surfaces. Prior art adhesion promoters include diacetone acrylamide (DAA) and isobutoxy methyl acrylamid (IBMA).
There exists a need for improved adhesion promoters, and the present invention is directed to photoimageable compositions having agents which promote their adhesion to metal surfaces.